Holey optical fiber with random pattern of holes and method for making same

ABSTRACT

A random array of holes is created in an optical fiber by gas generated during fiber drawing. The gas forms bubbles which are drawn into long, microscopic holes. The gas is created by a gas generating material such as silicon nitride. Silicon nitride oxidizes to produce nitrogen oxides when heated. The gas generating material can alternatively be silicon carbide or other nitrides or carbides. The random holes can provide cladding for optical confinement when located around a fiber core. The random holes can also be present in the fiber core. The fibers can be made of silica. The present random hole fibers are particularly useful as pressure sensors since they experience a large wavelength dependant increase in optical loss when pressure or force is applied.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/263,831, filed Nov. 3, 2008, now pending, which is acontinuation of U.S. patent application Ser. No. 10/863,805, filed Jun.9, 2004, now U.S. Pat. No. 7,444,838, which claims the benefit ofpriority from provisional application 60/515,447, filed on Oct. 30,2003. Related material is disclosed and claimed in U.S. patentapplication Ser. No. 11/929,058, filed Oct. 30, 2007, now U.S. Pat. No.7,567,742. The disclosures of all of the above-cited applications arehereby incorporated by reference in their entireties into the presentapplication.

FIELD OF THE INVENTION

The present invention relates generally to holey optical fibers andmethods for making holey optical fibers. More specifically, the presentinvention relates to a new technique for making holey optical fibershaving random patterns of holes. In the present method, a gas-generatingmaterial included in the fiber preform forms the holes as the fiber isdrawn.

BACKGROUND OF THE INVENTION

Holey optical fibers have microscopic holes or voids for guiding light.In holey fibers, the core is solid (e.g. SiO2) and is surrounded by anarray of holes containing inert gas or air. The light guided in theoptical fiber may be confined to the central core region by one of twobasic mechanisms. In the first mechanism, light is confined to thecentral core region by a refractive index difference between the coreand cladding material. In conventional solid glass fibers, therefractive index difference is produced by dopants in either the core orcladding material in order to raise or lower the refractive indices ofthese regions. In general it is desired for the core region to have ahigher refractive index than the cladding region. This can either beaccomplished by doping materials such as germanium or similar elementsin the core to raise the index or doping fluorine or similar in thecladding region to lower the refractive index. The index of the claddingregion can also be lowered by introducing porosity in that region. Themicroscopic holes have a much lower refractive index compared to thesolid core, so light is confined to the core. In the second type ofconfinement mechanism, the size and spacing of the holes is controlledin a very uniform and well defined pattern such that a photonic band gapis produced. The holes must be periodically spaced and carefullyarranged and maintained in the fiber to achieve the photonic band gapeffects. These fibers are often referred to as photonic crystal fibersowing to their period arrangement of air holes in the fiber. Themicroscopic holes provide unusual optical properties such as single-modeoperation over a wide wavelength range, low zero-dispersion wavelength,and highly controllable birefringence. As a result, holey optical fibersare expected to have a wide range of applications in optical sensors andtelecommunications.

Holey optical fibers are conventionally manufactured by stacking anarray of hollow silica tubes to form a preform. The tubes are carefullyarranged to control the spacing between them and to ensure thecrystalline arrangement. The preform is then heated and drawn intofibers as known in the art. The tubes generally experience a uniformscale reduction during drawing so that the tubes create the microscopicholes in the fiber.

One of the drawbacks of the conventional method for making holey opticalfibers is the complexity of assembling the stack of tubes. Also, thetube-stacking method cannot be used to produce fibers with random arraysof holes.

SUMMARY OF THE INVENTION

The present invention includes an optical fiber having a holey regionwith a random array of holes. In the present invention, the holes arecreated by gas generated during fiber drawing.

The holey region can be disposed around a fiber core, so that the holeyregion functions as a cladding.

The gas can be generated by nitride or carbide compounds. Siliconnitride and silicon carbide are exemplary gas generating materials.Carbides will typically produce carbon monoxide or carbon dioxide gas bydecomposition and oxidation of carbon.

The holes may be filled with the gas generated during fiber drawing. Thegas may be nitrogen, carbon monoxide, carbon dioxide or mixed nitrogenoxides, for example.

The random holes can have a uniform or nonuniform hole distribution.

The present invention includes a method for making the present randomhole optical fiber. In the method, a preform contains the gas generatingmaterial that produces gas bubbles when heated. The preform is heatedand drawn so that the gas bubbles are drawn into long holes. The preformmay comprise a glass powder mixed with the gas generating material.

The gas generating material may be provided in the form of a liquidprecursor. The liquid precursor may convert to a nitride or carbidematerial when heated.

Oxygen may be provided to the interior of the fiber preform so that thegas generating material is exposed to oxygen as it is heated.

The present invention includes a pressure sensor or force sensor havingthe present random hole optical fiber. The present random hole opticalfiber exhibits increased optical loss when in response to appliedpressure or force. Hence, the random hole fiber can be used as apressure or force sensor by monitoring optical loss in the fiber.

DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the present method for making random hole opticalfiber.

FIG. 2 shows a cross-sectional view of a random hole optical fiber.

FIG. 3 shows a cross-sectional view of a fiber having random holes inthe core.

FIG. 4 shows a cross sectional view of a fiber having random holes incombination with holes made from stacked tubes.

FIG. 5 shows a cross sectional view of a fiber having a non-uniformdistribution of random holes.

FIG. 6 shows a cross sectional view of a fiber having a non-uniformdistribution of random holes in which the random holes are confined byhollow tubes.

FIG. 7 shows a pressure sensor according to the present invention.

FIG. 8 shows plots of loss versus wavelength when 500 gram and 300 gramweights are rested on a random hole fiber.

FIG. 9 shows plots of loss versus wavelength when 4000, 2000, and 800gram weights are rested on a random hole fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

The present invention provides a holey optical fiber with a random arrayof holes. In the present invention, the holey optical fiber is made byincluding a gas-generating material in the fiber preform. In a typicalembodiment, the gas generating material is located around a solid core(e.g., made of solid SiO2). The gas generating material may be a nitrideceramic (e.g., silicon nitride, rare earth nitrides, AlN, TiN) orcarbide ceramic (e.g. silicon carbide, rare earth carbides) thatdecomposes at or close to the fiber drawing temperature (e.g., 1500-1600Celsius in the case of pure fused silica fibers). Decomposition producesgas bubbles (e.g., N₂, CO₂, carbon monoxide or nitrogen oxides) in themolten preform material as it is drawn. The gas bubbles are randomlydistributed and are drawn into long thin holes (or tubes) that remain inthe optical fiber. The holes reduce the refractive index in the regionsurrounding the solid core, and so provide light confinement. The numberand size of the holes can be influenced by the preform composition,drawing temperature, parent material particle size and drawing speed,among other factors. Though the holes have random locations, they canhave nonuniform distribution by nonuniformly distributing the gasgenerating material.

FIG. 1 illustrates the present method for making random-hole opticalfiber. In the present method, a fiber preform 18 comprises a silica tube20 filled with a holey region forming powder 22 and a solid silica rod24. The holey region forming powder 22 forms a fiber cladding and thesolid rod 24 forms a fiber core.

Heaters 26 heat the preform 18 so that it can be pulled to form a fiber28, as known in the art.

The holey region forming powder 22 comprises a mixture of a glassmaterial (e.g., high purity silica powder) and a gas-generating material(e.g., silicon nitride). The gas generating material produces a gas whenheated above the sintering temperature of the glass material. The gasgenerating material can produce gas by thermal decomposition or bychemical reactions (e.g. oxidation) with other components of the holeyregion forming powder, for example. The gas generated within the holeyregion forming powder 22 forms trapped bubbles 30 as the holey regionforming powder 22 sinters and softens. The bubbles 30 are stretched anddrawn into elongated tubes 32 as the fiber 28 is pulled. In preferredembodiments, the glass material is silica, and the gas generatingmaterial is a nitride or carbide ceramic.

FIG. 2 shows a magnified, cross sectional view of the present randomhole optical fiber. The fiber has a core 34 formed from the solid rod24, and a solid covering 36 formed from the tube 20. A holey region 38provides a cladding around the core 34. The holey region is formed fromthe holey region forming powder 22 and contains a large number(e.g., >50 or >200) of holes 40. The holes 40 are created from thebubbles 30. The holes 40 are preferably long tubes that may becentimeters, meters, or kilometers long, for example, depending upon thesize of the starting bubble in the preform and the additional gasgenerated (if any) during the drawing of the bubble into the tube. Theholes will typically have finely tapered ends, such that the diameter ofthe tube will reduce in size gradually as it reaches the end. The holesmay have typical diameters of 0.01-5 microns, for example, and may beeven smaller at the ends. The holes can be as large as tens of microns.The holes typically lack continuity, but new ones form as old ones endso that the holey region 38 has a relatively constant porosity. Theholes 40 are filled with gas and so tend to reduce the averagerefractive index of the cladding region 38. As a result, optical energywill be confined within the core 34 as known in the art. In preferredembodiments, the holes 40 are smaller than a wavelength of light in thefiber core, so that optical energy is minimally perturbed by individualbubbles 40. Very small holes (e.g., less than 100 nm in diameter) may berequired in applications where perturbation of the optical energy isundesirable.

The holey region forming powder 20 comprises mostly glass material(e.g., high purity silica powder) with a portion of a gas generatingmaterial. The gas generating material is preferably a ceramic (e.g.,nitride or carbide) that generates gas at or close to the temperaturerequired for fiber drawing. For example, the gas generating material cangenerate gas at temperatures in the range of about 1000-1600 C forsilica fibers. The gas generating material should generate gas attemperatures above the sintering temperature of the holey region formingpowder so that generated gas is trapped and cannot escape. The gas canbe generated by decomposition and/or oxidation, for example. The gasthat forms the bubbles 30 can be any gas, but is preferably a relativelyinert gas that does not interfere with desired light transmissionproperties of the optical fiber.

In a preferred embodiment, the gas generating material is siliconnitride. The silicon nitride can be a powder mixed into the holey regionforming powder 22. Alternatively, the silicon nitride can be a coatingon the particles of glass material. Silicon nitride is a preferredmaterial for generating the gas bubbles 30 because it produces arelatively large amount of gas at the drawing temperature, and becauseit oxidizes to SiO2 (a preferred fiber material) during drawing. Siliconnitride is a preferred gas generating material when silica is the glassmaterial.

Silicon nitride can be present in amounts less than about 1% by weightof the holey region forming powder when the balance is silica. Inoptical fibers manufactured by the present inventors, silicon nitridecomprises about 0.01-0.5%, or, more typically, 0.04-0.1% of the holeyregion forming powder by weight, with the balance of the holey regionforming powder being high purity silica. The amount of silicon nitridewill influence the porosity of the holey region. Larger amounts ofsilicon nitride will tend to produce higher porosity, and hence, arelatively lower average refractive index in the holey regions.

The porosity of the holey region can vary widely. For example, thepresent invention can produce porosities from less than 1% to 95% andhigher. Low porosities can be used in the cladding region ofindex-guiding fiber. High porosities can be used to reduce optical lossfrom Rayleigh scattering, for example.

While not wishing to be limited to a specific mechanism, it is believedby the present inventors that silicon nitride produces gas by oxidationduring drawing. Oxygen present in the holey region forming powderoxidizes the silicon nitride, producing SiO₂, and nitrogen or nitrogenoxides or some mixture thereof. Oxygen might also oxidize the nitrogento form mixed nitrogen oxides. The oxygen may be adsorbed on thesurfaces of holey region forming powder particles, may be trapped invoids during sintering, or may be dissolved in the silica. Oxygen boundto silica may also contribute to the oxidation. The nitrogen andnitrogen oxide gases may remain in the holes of the final, drawn fiber.

It is noted that oxygen or other gases (e.g., inert gases) may beincorporated into the holey region forming powder during drawing. Forexample, flowing oxygen gas into the holey region forming powder mayincrease the oxidation of the silicon nitride and gas generation. Thefiber preform can also be filled with ambient air. Solid oxygen sources(e.g., nitrates) can also be incorporated into the holey region formingpowder.

The holey region forming powder can have a wide range of particle sizes.Typical fibers made by the present inventors have employed 325 mesh and100 mesh silica powder and sub-micron diameter silicon nitride powder.Other particle sizes can also be used and may influence the size of theholes 40 or porosity of the holey region. The size of the powderparticles can affect the sintering temperature and amount of gasgenerated. If mixtures of powders are used, they should be thoroughlymixed before drawing, unless non-uniform hole distributions are desired.

It is noted that pre-oxidation of the gas generating material, drawingspeed, pulling force and drawing temperature may also influence theporosity of the holey regions or the size of the holes.

The glass material of the holey region forming powder is preferably highpurity silica. However, other glassy materials can be used instead. Forexample, fluoride-containing glasses, borosilicate glasses, or otheroptical glasses can be used instead of silica. These other glasses mayrequire different drawing conditions (e.g. different temperatures,absence of oxygen) and so may require use with specific gas generatingmaterials.

Silicon carbide can also be used as the gas generating material. Siliconcarbide tends to oxidize in the presence of oxygen at high temperature,forming SiO₂ and carbon monoxide or carbon dioxide. The carbon monoxideor carbon dioxide provides the bubbles 30. The oxygen source can beelemental oxygen present in the preform. Pure gaseous oxygen can beflowed into the preform to increase the amount of available oxygen.However, it has been empirically observed that silicon carbide tends toform less gas than silicon nitride, on a weight percentage basis. Forthis reason, the holey region forming powder may require more than 1% byweight silicon carbide (e.g., 1-5%) for adequate gas generation, whencombined with silica. Of course, the required amount of silicon carbidedepends upon the desired porosity and application of the optical fiber.

Many materials other than silicon nitride and silicon carbide can beused as the gas generating material. It is noted that, in general, thegas generating material should have the following characteristics:

1) The gas generating material should produce gas at a temperature at orabove the sintering temperature of the holey region forming powder. Ifthe gas generating material produces all of its gas at a temperaturebelow the sintering temperature of the powder, then gas will not betrapped within the preform, and bubbles will not be formed. Sinteringtemperature will be affected by heating duration, particle size,particle compaction, and particle surface chemistry. For example, 325mesh silica powder typically can be sintered at about 1000-1500 Celsius.Silicon nitride is suitable for use with silica because it continues toproduce gas at typical silica sintering temperatures.

2) The gas generating material should produce gases that do not impairdesired optical properties of the optical fiber. In many cases, anddepending upon the application, the reaction products of the gasgenerating material must not exhibit too high of optical losses to thepropagating signal. Silicon nitride meets this guideline because thenitrogen, nitrogen oxides and SiO₂ do not interfere with opticaltransmission significantly in most of the wavelength regions of interest(e.g., optical and near-infrared).

It is noted that many nitride materials can be used as thegas-generating material. Examples of possible nitride materials that canbe used include aluminum nitride, titanium nitride, rare earth metalnitrides (e.g. erbium nitride, nyodimuim nitride), and boron nitride.Other metal nitrides or intermetallic nitrides can also be used. Metalnitrides and intermetallic nitrides tend to decompose at hightemperature, or oxidize in the presence of oxygen, thereby forming gasbubbles of nitrogen or nitrogen oxides.

Also, many carbide materials can be used as the gas generating material.Carbide materials that can be used include aluminum carbide, titaniumcarbide, rare earth carbides and other metal or intermetallic carbides.Carbide materials tend to decompose and oxidize into carbon dioxide andcarbon monoxide in the presence of oxygen.

Also, nitrate and carbonate compounds may be used as the gas generatingmaterial. The nitrate or carbonate material should be a metal compound.For example, sodium nitrate or sodium carbonate can be used. Nitrateswill form nitrogen, nitrogen oxides, and possibly oxygen; carbonateswill form carbon dioxide, carbon monoxide and possibly oxygen. Thenitrate or carbonate material can be added to the preform as a powder,or an aqueous solution, for example. Other nitrates or carbonates thatcan be used include potassium nitrate or carbonate, rare earth nitratesand carbonates, and aluminum nitrate or carbonate.

Rare earth metal nitrides, carbides, nitrates and carbonates, inaddition to providing gas bubbles, can provide rare earth dopants (e.g.,erbium, neodymium, etc.) or other dopants with desirable opticalfunctions such as optical amplification, fluoresence or frequencyshifting. Other dopants can also be incorporated into the fiber. Usefuloptical properties of rare earth dopants and other dopants are known inthe art.

It is also noted that rare earth dopants or other dopants can beincorporated into the holey region forming powder as dopants in theglass material.

In another aspect of the invention, a liquid precursor material is usedto provide the gas generating material. The liquid precursor candecompose with heating to produce carbides or nitrides that subsequentlyrelease bubble-forming gas. In this embodiment, the liquid precursor ismixed with glass powder to form the holey region forming powder. Theholey region forming powder will be a slurry or paste in thisembodiment. The liquid precursor can be a polysilizane (e.g.polyureasilizane), alumoxane, or polyurethane, or other suitable liquidas know in the art to produce solid compounds that generate gases whenheated. Specifically, these liquid materials are known to form nitridesand carbides when heated. The choice of liquid precursor will in generalbe subject to the considerations addressed above, includingdecomposition temperature, gases produced and resulting oxidationprocesses. An advantage of using a liquid precursor is that the liquidforms coatings or particles of gas generating material with extremelyhigh uniformity. For example, the liquid precursor may form a thincoating of gas generating material on each particle of glass material inthe holey region forming powder. A coating on each particle will tend toproduce more uniform distribution of gas generating material compared toa mixture of particles. A highly uniform distribution of gas generatingmaterial will tend to create a highly uniform distribution of holes, andholes with smaller sizes. Uniform hole distribution and small hole sizeare typically preferred in optical fiber applications.

To control the amount of gas generating material created by the liquidprecursor, the liquid precursor can be diluted with a solvent. A highlydiluted liquid precursor material will tend to produce fewer bubbles andlower porosity in the optical fiber. Solvents that can be used includealcohol, chlorinated hydrocarbons, acetates, ethers, etc., dependingupon the liquid precursor used, with different solvents being suitablefor different precursors, as known in the art. 4h

FIG. 3 shows an alternative optical fiber that has holes in the coreregion 34. This fiber can be made by replacing the solid rod 24 of FIG.1 with a column of glass powder having a lower concentration of gasgenerating material. The glass powder can be disposed within a hollowtube so that it has a well defined diameter. The hollow tube could bemade of glass and remain during the draw or could be withdrawn beforethe fiber drawing occurs so that the tube is not incorporated into thefiber. Alternatively, the tube could be made from a thin polymericmaterial that vaporizes at low temperature. A possible advantage ofproviding holes in the core region is that it could reduce Rayleighscattering and optical loss by minimizing the amount of glass materialin the core. In order to reduce Rayleigh scattering as much as possible,high porosities may be desirable. For example, the core may have a 97%porosity and the cladding a 98% porosity.

Many other fiber structures are also possible in the present invention.FIG. 4, for example, shows a cross-section of an optical fiber withrandom holes generated by gas 40, in combination with an organized arrayof six holes 42 formed from drawn hollow glass tubes. This embodimentcan be created by stacking six hollow glass tubes around the solid rod34 in the preform of FIG. 1, as known in the conventional art of makingholey fibers. The space around the tubes is filled with holey regionforming powder.

FIG. 5 shows a cross-section of an optical fiber with a non-uniformdistribution of random holes 40. The fiber of FIG. 5 may have strongbirefringence and may have useful polarization maintaining properties.The fiber of FIG. 5 can be made by non-uniformly distributing gasgenerating material in the preform. Alternatively, the fiber of FIG. 5can be made by including large D-shaped solid glass rods in the preform.

FIG. 6 shows another embodiment of the invention in which holey regionsare confined within two drawn hollow glass tubes 44. Areas 46 outsidethe tubes 44 may be created from glass powder lacking gas generatingmaterial or having a smaller amount of gas generating material. Thefiber of FIG. 6 can be made by packing tubes 44 with holey regionforming powder before drawing.

It is noted that many different fiber structures can be made bycombining gas generating powder with glass powder, hollow glass tubes,and solid glass tubes. Holey regions can be localized by disposing gasgenerating material within a hollow glass tube. Solid regions can becreated from glass powder lacking gas generating material or from usingsolid glass elements. An infinite variety of structures are possiblewithin the scope of the present invention.

The present random hole optical fibers are pressure sensitive and can beused in pressure and force sensing applications, Specifically, therandom hole fiber experiences an increase in optical loss when pressureis applied in a direction orthogonal to the fiber axis, or when thepressure is hydrostatically applied.

The optical loss of the random hole fiber varies with wavelength. Theloss is generally greater for relatively short wavelengths (e.g.,wavelengths shorter than 600 nm) than for long wavelengths. However, thewavelength dependence of loss is complex and a function of the physicalstructure of the fiber.

The random hole fiber is sensitive to linear force, and to isotropichydrostatic pressure applied by a fluid medium. A linear force can beapplied by placing a weight on top of the fiber, for example.

FIG. 7 shows a simple pressure sensor according to the present inventionhaving a section of random hole optical fiber 50. The optical loss ofthe optical fiber is responsive to applied pressure 52. The appliedpressure can therefore be determined by monitoring the loss of the fiberwith an optical detector.

FIG. 8 shows plots of optical loss as a function of wavelength for arandom hole optical fiber. The fiber has a solid core and a holeycladding, like the fiber of FIG. 2. FIG. 8 illustrates the optical losswhen a 300-gram weight rests on a 5 centimeter length of the fiber, andthe optical loss when a 500-gram weight rests on a 5 centimeter lengthof the fiber. The optical loss increases substantially in the shortwavelength (i.e. less than 600 nm) as the weight is increased. Theoptical loss continues to increase as the weight is increased beyond 500grams, although this is not illustrated. Optical loss can be used todetermine pressure or force applied to the fiber.

FIG. 9 shows plots of optical loss as a function of wavelength for arandom hole fiber different from the fiber used to generate FIG. 8. Thefiber used to generate FIG. 9 has a solid core and a holey cladding,like the fiber of FIG. 2. FIG. 9 illustrates the optical loss when 4000g, 2000 g, and 800 g, weights rest on 2.85 centimeter lengths of thefiber. Also, the loss with zero pressure applied (zero weight) is alsoshown. From FIG. 9 it is clear that the optical loss increasesdramatically with increasing pressure.

It is noted that the optical loss due to pressure or applied force isrepeatable and does not exhibit hysteresis. Repeated tests confirm thatapplied pressure does not produce permanent alterations in the opticalloss of the fiber. Also, it is noted that the optical loss is relativelyinsensitive to temperature changes. These features make the presentrandom hole optical fibers ideal for applications in pressure and forcesensing applications. Also, the relative lack of pressure sensitivity inthe longer wavelengths provides a ready means for calibrating such asensor, and providing self-calibration during operation.

Additionally, it is noted that the pressure measurement provided byoptical loss variations is a distributed measurement. The optical lossis a function of the pressure magnitude in addition to the length offiber experiencing the pressure.

While not wishing to be limited to a specific mechanism, it is believedthat pressure induced loss in the random hole optical fibers is a resultof stress induced birefringence, optical tunneling, or highly localizedmicrobends. The random pattern of holes may create nonuniform structuraldeformation in the fiber, and therefore loss-inducing microbends.

The present random hole optical fiber can be used to sense force orpressure in a wide range of sensing applications. The fiber can be usedto detect pressure by monitoring the amount of optical loss detected.

The present method for making random hole optical fiber provides severalsignificant advantages including ease of fabrication, potential forcontinuous fiber drawing, and lower fabrication costs compared toconvention techniques for making holey fiber.

It will be clear to one skilled in the art that the above embodiment maybe altered in many ways without departing from the scope of theinvention. Accordingly, the scope of the invention should be determinedby the following claims and their legal equivalents.

What is claimed is:
 1. An optical fiber comprising: a core composed of aglass of a refractive index; and a cladding region composed of the sameglass of the same refractive index, wherein the cladding region containstubes which are random in diameter, length and radial position withinthe cladding region, and wherein the tubes taper on the ends.
 2. Thefiber of claim 1, wherein the ends of the tubes are tapered such thatsignificant scattering of light does not occur.
 3. The fiber of claim 1,wherein an optical loss of the fiber is less than 1 dB/m.
 4. The fiberof claim 3, wherein the optical loss as measured at 1550 nm is less than0.5 dB/m.
 5. The fiber of claim 3, wherein the optical loss as measuredat 1310 nm is less than 0.5 dB/m.
 6. The fiber of claim 3, wherein theoptical loss as measured at 850 nm is less than 0.5 dB/m.
 7. The fiberof claim 3, wherein the optical loss as measured in the range between1550 and 1700 nm is less than 0.5 dB/m.
 8. The fiber of claim 3, wherebythe optical loss induced by a half inch diameter bend in the fiber isless than 0.5 dB.
 9. The fiber of claim 3, wherein the optical lossinduced by a half inch diameter bend in the fiber is less than 0.5 dBand more preferably less than 0.1 dB when measured at a wavelength of850 nm.
 10. The fiber of claim 3, wherein the optical loss induced by ahalf inch diameter bend in the fiber is less than 0.5 dB and morepreferably less than 0.1 dB when measured at a wavelength of 1310 nm.11. The fiber of claim 3, wherein the optical loss induced by a halfinch diameter bend in the fiber is less than 0.5 dB and more preferablyless than 0.1 dB when measured at a wavelength of 1550 nm.
 12. The fiberof claim 3, wherein the optical loss induced by a half inch diameterbend in the fiber is less than 0.5 dB and more preferably less than 0.1dB when measured in thea wavelength range from of 1550-1700 nm.
 13. Thefiber of claim 1, wherein the core of the fiber when viewed incross-section is located at a center of the fiber, such that a center ofthe core is coincident with the center of the fiber.
 14. The opticalfiber of claim 1, wherein optical confinement is achieved byincorporation of the tubes into the fiber such that an interface betweena solid central glass region and a surrounding region containing thetubes defines an interface between the core and cladding region.
 15. Thefiber of claim 14, wherein the tubes in the cladding region lower anaverage effective refractive index of that region, thereby producing adifference in refractive index between the core and cladding regionnecessary for optical confinement to be achieved in the core of thefiber.
 16. The fiber of claim 15, wherein the tubes are random indiameter, random in length and random in spatial position within thecladding region and such that if the tubes were not present, no claddingwould exist.
 17. The fiber of claim 16, wherein the tubes in thecladding region end by forming a tapered region such that an innerdiameter of the tube decreases continuously to a very small valuerelative to an initial diameter of the tube.
 18. The fiber of claim 17,wherein the tubes are adiabatically tapered.